This invention relates to an electrochromic display device having a selectively colorable transition metal oxide film and an acidic aqueous electrolyte, wherein the film reacts with electrolyte protons in the presence of an applied electric field to change from a transparent state to a colored state. More particularly, this invention relates to extending the useful lifetime of an electrochromic display device by minimizing the dissolution of the transition metal oxide film into the acidic aqueous electrolyte.
In a conventional electrochromic display device, the desired display is developed by applying an electric field to an electrochromic transition metal oxide film to cause the film to change its electromagnetic radiation-transmitting properties. Several suitable electrochromic transition metal oxides are known, tungsten oxide typically being preferred because it forms a highly visible display. The tungsten oxide is vapor-deposited onto a transparent tin oxide electrode, which has in turn been applied to one surface of a supporting glass plate. The deposited film is contacted with an acidic aqueous electrolyte, for example, by 10% by volume H.sub.2 SO.sub.4 solution containing a white pigment. The electrolyte is in turn in contact with a suitable counterelectrode. When the transparent electrode is connected to a negative battery terminal and the counterelectrode is connected to the positive terminal, the film reacts with protons from the electrolyte to change colors in accordance with the following empirical half-reaction: EQU xe.sup.- +xH.sup.30 +WO.sub.3 (colorless).rarw..fwdarw.HWO.sub.3 (blue)
wherein the extent of reaction x is controlled by the current entering the film and determines the depth of coloration. Thus, the film changes from a transparent state to a blue state, designated tungsten bronze. When viewed through the supporting glass plate, a tungsten bronze (blue) display is seen against a contrasting white background provided by the pigmented electrolyte. The tungsten bronze state persists after the electric field is discontinued. However, by reversing the polarity of the electrodes, the reaction is reversed, the electrochromic film is bleached, and the display is erased.
A major problem encountered with conventional electrochromic display devices is that the tungsten oxide film dissolves in the aqueous electrolyte. Film dissolution occurs even as the device sits on the shelf and is substantially increased by the cyclic coloring and bleaching of the film to develop and erase the display. Thus, the device has a short shelf lifetime and an even shorter cycle lifetime. Devices employing other electrochromic transition metal oxide films also encounter problems of film dissolution into the aqueous electrolyte.
It has been proposed to reduce film dissolution by employing an electrolyte consisting of a suitable acid dissolved in an organic solvent, such as glycerine, since transition metal oxides are substantially less soluble in organic solvent than in water. However, the use of an organic-base electrolyte unacceptably increases the time required to color or bleach the film. It has also been proposed to place a porous membrane permeated with electrolyte adjacent the electrochromic film. To permit protons to flow into and out of the film to effect the desired color-changing reaction, electrolyte in the membrane pores at the film interface contacts the film. Where the electrolyte contacts the film, dissolution continues to occur, thereby reducing the device lifetime.
Therefore, it is an object of this invention to improve the shelf and cycle lifetimes of an electrochromic device having an electrochromic transition metal oxide film and an acidic aqueous electrolyte by preventing direct contact of the film and the electrolyte without significantly restricting the flow of protons therebetween. The ready flow of protons effects the desired electrochromic reactions, but the lack of film-electrolyte contact reduces film dissolution and thereby extends the useful device lifetime.
It is a further object of this invention to provide a nonporous, proton-conductive, acid-resistant polymer barrier to separate a transition metal oxide film and an aqueous acidic electrolyte in an electrochromic display device to inhibit the film from dissolving into the electrolyte. The barrier readily conducts electrolyte protons that react to color and bleach the film, but improves the useful lifetime of the device by reducing film dissolution.